Fireforming

[Hikinghunter]

Howdy. I'm doing load development for my new custom build. When I installed the barrel, I headspaced within .001 of my virgin brass length. As such, I'm not seeing any shoulder growth in my once-fired brass. I assume this makes fire-forming moot for this brass/rifle? Or is the brass still fire-forming to the chamber, just not lengthening?

 

[Tiny Tim]

Alot of times it will take 2,3, or possibly more firings before you have to bump the shoulders back. It's likely still forming to the chamber.

 

[Hikinghunter]

Alot of times it will take 2,3, or possibly more firings before you have to bump the shoulders back. It's likely still forming to the chamber.

Ok, but why would the shoulders move at all if there isn't much room for the brass to expand inside the chamber? If I put a piece of tape on the back of the brass the bolt won't close, so maybe 1thou of headspace.

 

[GSRetrievers]

Shot the same brass without hitting the shoulder sizing, you'll find the answer to your question in the next firing or two.

 

[Mikecr]

Ok, but why would the shoulders move at all if there isn't much room for the brass to expand inside the chamber?

WHY is always a good inquiry.
But I don't know..
I just know it does, and does so evermore with lower shoulder angles.

If this is a barrel nut setup, you may have less growth than shouldered barrels, because the nut pre-pulls ALL threading away from the bolt..
This, engaging more threading at once (think stud connection -vs- bolt).

 

[MagnumManiac]

My 260AI, custom chamber, shows no brass movement for over 6 firings. The chamber has '0' headspace, body has .001" above unfired brass and the neck is the only 'loose' area in the chamber. It has .0025" clearance in the neck on fired brass.
I have not yet reached chamber end on this Lapua brass, and have only trimmed once to square up case mouths, case growth is virtually zero in every aspect.
After 6 firings, I am just kissing the shoulder so the bolt falls with gravity, difficult to measure, but it is around .001" or less of bump.

I am very happy with this outcome, brass will last indefinitely.

Edit to add:
My 6.5x47 is the same as above and was the inspiration for this.

Cheers.

 

[FEENIX]

Alot of times it will take 2,3, or possibly more firings before you have to bump the shoulders back. It's likely still forming to the chamber.

Shot the same brass without hitting the shoulder sizing, you'll find the answer to your question in the next firing or two.

^^^This!^^^

Ok, but why would the shoulders move at all if there isn't much room for the brass to expand inside the chamber? If I put a piece of tape on the back of the brass the bolt won't close, so maybe 1thou of headspace.

If you feel strongly that your brass is fully fire-formed, go for it.

 

[HandgunHTR]

Just wanted to add that it isn't just the shoulders that grow. The web of the cartridge will expand to fit your chamber as well and that could take 2-3 firings as well.

That being said, I have found that an accurate load while fireforming or withing the first couple of firings on a piece of brass is still accurate after it is fully formed. Just wait until then to settle on your final powder charge and seating depth. Chances are it will be within a couple tenths of a grain and 0.010", respectively.

 

[MagnumManiac]

Ok, but why would the shoulders move at all if there isn't much room for the brass to expand inside the chamber? If I put a piece of tape on the back of the brass the bolt won't close, so maybe 1thou of headspace.

The brass is moving, but due to springback, it is virtually immeasurable. If you only neck size, you will find a point where it gets tight closing the bolt because brass expands ever so slightly larger beyond springback.
The number of firings for this to occur is not known until it happens.
Lapua brass seems to take many firings for this to happen, Norma brass takes far less due to it's thinner web and pressure needs to be run less for case life. Norma brass has the most capacity and therefore pressure is reduced, but people run it full throttle and primer pockets loosen early.

Cheers.

 

[Phil Rizzo]

MM, do zero headspace chambers tend to pressure up any earlier than one with room for say .002 growth?

 

[MagnumManiac]

MM, do zero headspace chambers tend to pressure up any earlier than one with room for say .002 growth?

Yes, but I expect it. My 264WM A191 based chamber and .220" long throat gets the same book velocity with less powder. If calculating the gains from a 29.5" barrel.
All of my 416 Rigby Improved cases exceed Weatherby numbers, this is due to the shoulder angles of 40° & 45° with long throats and tight chambers. I have pressure tested these rounds multiple times, I do not exceed 65,000psi, nor do I see pressure spikes if pressure is help back slightly. US869 is notorious for being nice, then not being nice…suddenly. RE22 is another.

Cheers.

 

[Phil Rizzo]

Ive culled out any powder that acts up, to the best of my knowledge.
Including R26.